1. Field of the Invention
The present invention relates to a method for regenerating a caustic (or alkali) etch solution after being used in the caustic etching of aluminum and the alloys containing, as a main constituent, aluminum such as duralumin (which is referred to as "aluminum and the alloys thereof" hereinbelow).
2. Description of the Prior Art
In the case where aluminum and the alloys thereof are anodized, caustic etching using an aqueous sodium hydroxide solution is generally carried out as a pre-treatment. During the etching, since aluminum is dissolved into the etch solution in the form of sodium aluminate, the concentration of the free sodium hydroxide decreases with as the etching proceeds, and finally, etching becomes impossible. The etching waste liquid thus formed has been neutralized heretofore, and large amounts of sludge generally having a high water content are generated and cause environmental pollution. Furthermore, the regeneration of the caustic etch solution has been heretofore carried out by partially using the principle of the Bayer method. However, this regeneration is only effective in the case where the etch solution already contains aluminum hydroxide in a supersaturated condition. However, in the case where the aluminum hydroxide is contained in the etch solution in an unsaturated condition or a slightly supersaturated condition, the regeneration can not be satisfactorily effected.
Recently, Japanese Patent Publication No. 51-22901/1976 has disclosed a new method in which, by utilizing the principle of the Bayer method, a etch solution is first diluted with water to thereby precipitate aluminum hydroxide and the resultant liquid is separated and concentrated and, then, the concentrated liquid is returned to an etching vessel. However, since the etch solution is diluted with a large amount of a washed water (i.e. 3-8 times by volume) to recover the sodium hydroxide which is discharged into the washed water, an extremely large amount of heat energy is required for the concentration of the diluted etch solution, and, therefore, this method is not advantageous from an economical point of view. Furthermore, the prevention of the accumulation of impurities in the system due to the circulation of the recovered etch solution is not taken into account.